Ceramic parts made of silicon nitride, silicon carbide or aluminum oxide combine high strength at temperatures up to 1500.degree. C. with a relatively high fracture toughness and a high thermal shock resistance. For this reason they are already being widely used in apparatus construction, as heat exchangers or in engine construction. To manufacture large numbers of parts with complex shapes, increasing use is made of the injection molding process. This is done for example by mixing the ceramic powders with thermoplastics and processing the compositions, which have a ceramic powder content of more than 50% by volume, into green parts by injection molding. Thereafter the polymer matrix is removed from the green parts, for example by pyrolysis, and the green parts are then sintered to give the actual ceramic structures. Extrusion processes make it possible to make pipes, profiles or ribbons.
It is known that crystalline silicon nitride or silicon carbide powder is sinterable to mechanically stable articles only in the presence of sinter aids. Such sinter aids for silicon nitride are aluminum oxide, boron oxide, magnesium oxide and/or rare earth oxides, preferably yttrium oxide, in amounts of from 2 to 10% by weight, based on the silicon nitride. Silicon carbide is sintered not only with about 1% by weight of carbon but also with either 1% by weight of aluminum oxide or 1% by weight of boron oxide. In the case of aluminum oxide 30 small amounts - less than 1% by weight - of silicon dioxide or magnesium oxide are used to avoid particle boundary growth. According to the latest school of thought, sinter aids combine with the boundary regions of silicon nitride powder particles to form an amorphous glassy phase in which inter-particulate mass transfer and hence the sintering together take place. Ideally, the silicon nitride particles should be surrounded for this purpose by a thin boundary zone of sinter aids. Customarily, the silicon nitride particles have dimensions of 1 .mu.m, and the sinter aid particles are roughly the same size. This results in a poor distribution of sinter aids and hence in local excess and deficient concentrations To make up for deficient concentration, more sinter aid is added to a batch than actually necessary. This increases the proportion of the intergranular glass phase in the sintered part. However, the increase in the glass phase content means a decrease in the temperature resistance, corrosion resistance, the resistance to subcritical crack growth and creep resistance.
One way of applying sinter aids uniformly comprises for example mixing the ceramic powder with a metal alkoxide dissolved in an organic solvent, drying the mixture and then hydrolyzing the metal alkoxide. The process has the disadvantage that many alkoxides are volatile, which leads to handling problems (DE-A-3,637,506). Another method comprises treating the ceramic powders with aqueous solutions of salts of the sinter aid which can be converted thermally into the corresponding oxides.
It is an object of the present invention to provide a process for preparing thermoplastic compositions filled with ceramic powders and oxidic sinter additives which avoids the disadvantages of existing processes and makes it possible to prepare in one operation thermoplastic compositions which contain ceramic powders modified with oxidic sinter additives.